@mixin apply-to-ie6-only {
    * html {
        @content;
    }
}
@include apply-to-ie6-only {
    #logo {
        background-image: url(/logo.gif);
    }
}
//  output:
//  * html #logo {
//     background-image: url(/logo.gif);
//  }
// 这里的@content就是代表的
// #logo {
//     background-image: url(/logo.gif);
// }
  
